The cochlea is a remarkable sensor: a living cochlea can reliably detect sounds that cause motions of the stapes on the order of picometers, is capable of high-quality frequency analysis (Q10dB > 600), and compresses the large dynamic range (120 dB) of hearing into the considerably smaller dynamic range (20- 50 dB) of neurons. It is now widely accepted that an active mechanical amplification process underlies these remarkable properties. However, there is considerable debate about the nature of the amplifier. While information on the cellular and molecular basis of hearing is increasing rapidly, there is still little understanding of how the components interoperate to generate the remarkable properties of hearing. To date, the most successful experimental studies of cellular motions in living cochleae have used optical methods. However, current methods, such as laser Doppler vibrometry and video microscopy are limited by the low reflectivities of cochlear structures and the limited optical access provided by the intact cochea. The objective of this grant is to develop and apply a new tomographic imaging and motion measurement technique capable of determining the three-dimensional motions of all structures in a living, intact cochlea. Optical coherence tomography (OCT) will be used to obtain high-resolution images of the cochlea. Images will be acquired through a narrow opening similar to that used in laser Doppler vibrometry methods, or possibly directly through bone. Sequences of stroboscopic images will be generated with synchronous demodulation of the OCT detector signal during acoustic stimulation. Computer vision algorithms (similar to those used in video microscopy methods) will be used to determine motions with nanometer resolution. This technique will be applied to image and measure three-dimensional motions of the internal microstructure of an intact cochlea, including the basilar membrane, reticular lamina, tectorial membrane, and outer hair cells.